Static distortions of a cholesteric planar structure induced by magnetic or ac electric fields

Abstract

We discuss the threshold conditions for the nucleation of a static Helfrich distortion of a planar cholesteric structure under a magnetic field or an ac electric field applied parallel to the helical axis. In opposition to the case of nematics, the magnetic threshold in cholesterics is definitely lower than the threshold field for a transition of the Freedericks type. Under ac electric fields, the behavior of the threshold field with frequency is very similar to the one observed in nematics and depends mainly on the sign of the molecular dielectric anisotropy εa. For εa > 0, the threshold field is practically independent of frequency. For εa < 0, the threshold field diverges for a critical frequency proportional to the charge relaxation frequency. All these theoretical derivations seem to be in good agreement with recent experimental results.